CO2-rich Fluids Research Articles

Deep pre-eruptive storage of silicic magmas feeding Plinian and dome-forming eruptions of central and northern Dominica (Lesser Antilles) inferred from volatile contents of melt inclusions

Volatiles contribute to magma ascent through the sub-volcanic plumbing system. Here, we investigate melt inclusion compositions in terms of major and trace elements, as well as volatiles (H2O, CO2, SO2, F, Cl, Br, S) for Quaternary Plinian and dome-forming dacite and andesite eruptions in the central and the northern part of Dominica (Lesser Antilles arc). Melt inclusions, hosted in orthopyroxene, clinopyroxene and plagioclase are consistently rhyolitic. Post-entrapment crystallisation effects are limited, and negligible in orthopyroxene-hosted inclusions. Melt inclusions are among the most water-rich yet recorded (≤ 8 wt% H2O). CO2 contents are generally low (< 650 ppm), although in general the highest pressure melt inclusion contain the highest CO2. Some low-pressure (< 3 kbars) inclusions have elevated CO2 (up to 1100–1150 ppm), suggestive of fluxing of shallow magmas with CO2-rich fluids. CO2-trace element systematics indicate that melts were volatile-saturated at the time of entrapment and can be used for volatile-saturation barometry. The calculated pressure range (0.8–7.5 kbars) indicates that magmas originate from a vertically-extensive (3–27 km depth) storage zone within the crust that may extend to the sub-Dominica Moho (28 km). The vertically-extensive crustal system is consistent with mush models for sub-volcanic arc crust wherein mantle-derived mafic magmas undergo differentiation over a range of crustal depths. The other volatile range of composition for melt inclusions from the central part is F (75–557 ppm), Cl (1525–3137 ppm), Br (6.1–15.4 ppm) and SO2 (< 140 ppm), and for the northern part it’s F (92–798 ppm), Cl (1506–4428 ppm), Br (not determined) and SO2 (< 569; one value at 1015 ppm). All MIs, regardless of provenance, describe the same Cl/F correlation (8.3 ± 2.7), indicating that the magma source at depth is similar. The high H2O content of Dominica magmas has implications for hazard assessment.

Global-scale control of extensional tectonics on CO2 earth degassing

Earth degassing of CO2-rich fluids has been proven to contribute significantly to the global carbon budget. The presence of ubiquitous outgassing reveals some degree of permeability of the crust that often coincides with seismically active zones. In this study, we took advantage of the most recent global geological datasets to better understand earth degassing and how it correlates with tectonic regimes. Here we use an ad hoc point pattern analysis to show that there is a spatial correlation between CO2 discharges and the presence of active fault systems, in particular with those characterized by a normal slip type. Seismic data demonstrate the existence of a positive spatial correlation between gas discharges and extensional tectonic regimes and confirms that such processes would play a key role in creating pathways for the rising gases at micro- and macro-scales, increasing the rock permeability and connecting the deep crust to the earth surface.

CO2-rich fluid inclusions in mantle xenoliths from Barombi-Mbo and Nyos maars (Cameroon volcanic line, central Africa): Upper mantle equilibria and history of magma ascension

A detailed study of fluid inclusions hosted within minerals of mantle xenoliths from the Barombi-Mbo (BM) and Nyos maars provides information on the trapping conditions of fluids and subsequent upper mantle equilibria and the history of the host en route to the surface. These porphyroclastic to protogranular spinel lherzolites and their fluid inclusions were investigated using petrography, microprobe, microthermometry and Raman Microspectroscopy. They are mostly composed of olivine (Fo93-88), orthopyroxene (En91-88Fs11-8Wo2.2-0.8), Cr-diopside, aluminous spinel (Cr#: 0.32–0.08) and occasional accessory pargasite and labradorite. Thermobarometric calculations based on the mineral chemistry of the BM xenoliths indicate temperatures of 1205 to 800 °C at pressures of 23 to 14 kbar while the results on Nyos xenoliths give temperatures of 1134 to 1044 °C at pressures of 12 to 9 kbar consistent with mantle depths of 75–42 km to BM and 36-27 km to Nyos. Olivine and pyroxene crystals contain abundant CO2-rich inclusions (48-1 μm). In Nyos, Raman spectra allow the detection of CO2 and unknown species containing NH2 while CO2 and traces of nahcolite (NaHCO3) were detected in fluid inclusions in the Barombi-Mbo mantle xenoliths. The melting point (Tm) of CO2 varies from −56.6 to −58.0 °C and from −56.8 to −63.0 °C respectively in BM and Nyos xenoliths. 95% of inclusions homogenized in the liquid phase (ThL; L + V →L) and 5% in the vapor phase (ThV; L + V →V) at temperatures ranging from +31.1 to −48.1 °C in BM xenoliths, and from +31.1 to −38.1 °C in Nyos xenoliths. Densities of CO2 inclusions in minerals range from 1.1 to 0.35 g/cm3 at Nyos, and from 1.15 to 0.2 g/cm3 at BM. The high fluid pressures deduced and the corresponding depths are 11 to 8 kbar (33-24 km) at Nyos and 12 to 10 kbar (36-30 km) at BM. Taking into account the post-trapping stretch, the corrected inclusions densities and corresponding pressures and depths set the minimum xenoliths entrainment depths to 50 km beneath BM and 36 km under Nyos maars. Fluid inclusion pressures also reveal that the Moho and Conrad discontinuities lie at depths of 24 and 15 km and of 30 and 13.5 km, respectively at Nyos and BM. The estimated Moho depths are consistent with the geophysical observations along the CVL. Low densities and relative depths probably indicate short magma stops in intermediate magmatic chambers. Before eruption, the last short magma storage and fluid mixing occurred at a depth of less than 1.8 km at BM and 3 km at Nyos.

Tracing olivine carbonation and serpentinization in CO2-rich fluids via magnesium exchange and isotopic fractionation

Chemical exchange between seawater and the oceanic crust is thought to play a significant role in the regulation of the global magnesium (Mg) cycle, yet relatively little is known about the rates and mechanisms of Mg exchange in these crustal environments. In this study we experimentally characterize the extent, and nature, of Mg isotope fractionation during the carbonation and serpentinization of olivine (one of the principal minerals found in ultramafic rocks) under hydrothermal conditions. Olivine alteration was found to be incongruent, with the reactant fluid composition varying according to the extent of olivine dissolution and the precipitation of secondary minerals. In mildly acid water (pH ∼ 6.5), olivine dissolved to form Mg-Fe carbonate solid solutions and minor chrysotile. Upon carbonation and a decrease of CO2 in the water, the pH increased to >8, with chrysotile and brucite becoming the dominant alteration minerals. The Mg-rich carbonates preferentially incorporated lighter Mg isotopes, resulting in a ∼0.5‰ increase of the δ26Mg composition of the fluid relative to olivine during the initial carbonation and serpentinization reactions. This was followed by a decrease in δ26Mg under higher pH conditions associated with the formation of brucite. Our experimental and modeling results therefore demonstrate that the δ26Mg composition of fluids involved in olivine alteration reflect the type and quantity of secondary Mg minerals formed, which in turn depend on the pH and CO2 concentration of the water. Comparison of these results with natural groundwaters and geothermal waters from basaltic terrains indicate that the δ26Mg composition of natural waters are likely to also be controlled by mafic rock dissolution and the preferential incorporation of isotopically light Mg into carbonates and isotopically heavy Mg into Mg-Si minerals. Together, these findings improve our understanding of Mg isotope systematics during water-rock interaction, and suggest that δ26Mg may be a useful tool for tracing reactions that are critical to geological CO2 sequestration.

Rapid migration of CO2-rich micro-fluids in calcite matrices

The transport of supercritical fluids is a determining factor for several geological processes and fundamental in predicting natural resource accumulation and distribution. Calcite, ubiquitous in most geological environments, may contain supercritical CO2 trapped under the form of fluid inclusions that may move through grain boundaries affecting the rock physical properties. However, despite macroscopic evidence for this process, until recent it was not possible to characterize this process at the nano-scale due to the difficulty of such observations. In this study, we report nanometer-scale observations on calcite crystal surfaces and demonstrate that stress with absence of visible deformation produces fluid leakage from fluid inclusions. Atomic Force Microscopy scanning experiments on freshly cleaved calcite crystals containing visible fluid inclusions revealed the spontaneous formation of nanometer-scale hillocks on flat crystal terraces in only a few minutes, without evidence of surface dissolution. The fact the hillocks formed on flat surface in a short time was unexpected and suggests deposition of material from the inner crystal to the surface through small-scale fluid migration. We estimated the rate of this fluid mobility is by several orders of magnitude higher than the diffusion rate through vacancies estimated in calcite crystals showing that CO2–rich fluids through micro-pore and nano-pore spaces is in reality much higher than previously assumed using current predictive models.

Ore genesis and fluid evolution of the Kaladawan South Zn–Pb–Cu ore field, eastern Altyn Mountains (NW China): Evidence from fluid inclusions, H–O isotopes and geochronology

The newly-discovered Kaladawan South Zn–Pb–Cu orefield in eastern Altyn Mountains is composed of five Zn–Pb(–Cu) deposits, with the largest one containing 414.4 thousand tonnes (kt) Zn, 265.2 kt Pb and 149.2 kt Cu. Stratiform/lenticular orebodies are hosted by the Ordovician volcanic–sedimentary sequences subjected to greenschist-facies metamorphism. Two major ore-forming stages with significant cross-cutting relationship are identified at Kaladawan South: Stage (I) of the earlier massive, banded, disseminated barite-rich sulfide ores, and Stage (II) of the later polymetallic sulfide–quartz veins with economically less importance. Fluid inclusions (FIs) locked in quartz of Stage I massive sulfide ores are homogenized completely at 226–354 °C with calculated seawater-like salinities of 2.7–4.2 wt% NaCl equiv. These characteristics of ore-forming fluids coupled with ore textures of typical VMS deposits, are interpreted to be originated from the modified seawater by sea-floor volcanic-hydrothermal activities. In compared, the Stage II FIs yielded lower homogenization temperatures ranging 251 °C–357 °C with higher salinities of 0.8–14.4 wt% NaCl equiv. Laser Raman analyses identified large amounts of CO2 ± (CH4–N2) in the hydrothermal fluids of Stage II. The δ18OH2O values (0.5–5.7‰) and δDH2O values of (−68.1‰ to −32.3‰) of Stage II quartz are plotted in the range of metamorphic fluids in the D–O isotopic diagram, which indicating that the CO2-rich and mesothermal fluids are metamorphic-derived. The phlogopite paragenetic in the Stage II polymetallic vein yielded a 40Ar/39Ar plateau age of 435.4 ± 3.9 Ma, similar to the age of the metamorphic muscovite (451.0 ± 4.1 Ma) from the ore-hosting schist. Based on the ore deposit geology, mineral assemblages and fluid geochemistry, the Kaladawan South Zn–Pb–Cu mineralization is attributed to be an Ordovician VMS-type deposit overprinted by the Silurian metamorphic fluids.

Mechanical-Chemical-Mineralogical Controls on Permeability Evolution of Shale Fractures

We report experimental observations of permeation of CO2-rich aqueous fluids of varied acidic potential (pH) on three different shales to investigate mechanical, chemical, and mineralogical effects on fracture permeability evolution. Surface profilometry and SEM-EDS (scanning electron microscopy with energy-dispersive X-ray spectroscopy) methods are employed to quantify the evolution in both roughness on and chemical constituents within the fracture surface. Results indicate that, after 12 hours of fluid flow, fracture effective hydraulic apertures evolve distinctly under different combinations of shale mineralogy, effective stress, and fluid acidity. The evolution of roughness and transformation of chemical elements on the fracture surface are in accordance with the evolution of permeability. The experimental observations imply that (1) CO2-rich aqueous fluids have significant impact on the evolution of fracture permeability and may influence (and increase) shale gas production; (2) shale mineralogy, especially calcite mineral, decides the chemical reaction and permeability increasing when CO2-rich aqueous fluids flow through fractures by free-face dissolution effect; (3) clay mineral swelling reduces fracture aperture and additively calcite pressure solution removes the bridging asperities, which are the main reasons for fracture permeability decrease; (4) competition roles among clay mineral swelling, mineral pressure solution, and free-face dissolution determine how fracture permeability changes. Furthermore, a multiple parameter model is built to analyze effective hydraulic aperture evolution in considering above three mechanisms, which provide a reference to forecast fracture permeability evolution in shale formations.

Assessing the Validity of Negative High Field Strength-Element Anomalies as a Proxy for Archaean Subduction: Evidence from the Ben Strome Complex, NW Scotland

The relative depletion of high field strength elements (HFSE), such as Nb, Ta and Ti, on normalised trace-element plots is a geochemical proxy routinely used to fingerprint magmatic processes linked to Phanerozoic subduction. This proxy has increasingly been applied to ultramafic-mafic units in Archaean cratons, but as these assemblages have commonly been affected by high-grade metamorphism and hydrothermal alteration/metasomatism, the likelihood of element mobility is high relative to Phanerozoic examples. To assess the validity of HFSE anomalies as a reliable proxy for Archaean subduction, we here investigate their origin in ultramafic rocks from the Ben Strome Complex, which is a 7 km2 ultramafic-mafic complex in the Lewisian Gneiss Complex of NW Scotland. Recently interpreted as a deformed layered intrusion, the Ben Strome Complex has been subject to multiple phases of high-grade metamorphism, including separate granulite- and amphibolite-facies deformation events. Additional to bulk-rock geochemistry, we present detailed petrography, and major- and trace-element mineral chemistry for 35 ultramafic samples, of which 15 display negative HFSE anomalies. Our data indicate that the magnitude of HFSE anomalies in the Ben Strome Complex are correlated with light rare earth-element (LREE) enrichment likely generated during interaction with H2O and CO2-rich hydrothermal fluids associated with amphibolitisation, rather than primary magmatic (subduction-related) processes. Consequently, we consider bulk-rock HFSE anomalies alone to be an unreliable proxy for Archaean subduction in Archaean terranes that have experienced multiple phases of high-grade metamorphism, with a comprehensive assessment of element mobility and petrography a minimum requirement prior to assigning geodynamic interpretations to bulk-rock geochemical data.

Pore pressure and reservoir quality evolution in the deep Taranaki Basin, New Zealand

The Palaeocene fluvial to shallow marine sandstones of the Farewell Formation are an important proven hydrocarbon reservoir in the Taranaki Basin, New Zealand. The Kapuni Deep-1 well was drilled to target Farewell Formation sandstones in the deeper overpressured (>5000 m, >3000 psi/21 MPa) sections of the onshore Manaia Graben of the Taranaki Basin. However, the Farewell Formation sandstones display anomalously low measured helium porosities (1–4.5%) and intergranular volumes (6–11%) even for their present-day, close to maximum, burial depth (c.5000 m). One dimensional burial history modelling demonstrates initial rapid burial leading to significant porosity reduction via mechanical compaction, enhanced by poor sorting, angular grain morphology and the presence of ductile grains, which allowed efficient packing and plastic deformation. Low intergranular volume (IGV), anhedral nature of quartz overgrowths, a general lack of fluid inclusions, and poor crystalline clay mineral content indicate that early burial compactional processes significantly influenced reservoir quality. The lack of mudstone to siltstone grade lithologies within the overlying Eocene section inhibited the early or shallow onset of overpressure in the Farewell Formation. One dimensional basin modelling has shown that rapid Pliocene subsidence related to exceptionally high sedimentation rates generated overpressure through disequilibrium compaction in the overlying Oligocene to early Miocene section during the past 6 Ma. However, the late and deep (>3000 m) onset of overpressure had no effect on arresting porosity loss in the Farewell Formation. Continued compaction of Farewell Formation sandstones after dissolution of early carbonate cements of CO2 rich fluids creates zones of extremely low permeability, which have the potential to act with interbedded shales to form pressure seals as seen in the Kapuni Field.

Charge history of CO2 in Lishui sag, East China Sea basin: Evidence from quantitative Raman analysis of CO2-bearing fluid inclusions

Abstract The accumulation of carbon dioxide in natural gas reservoirs is common, and CO2 gas fields have been widely discovered in the world. In the last decades, three CO2 pools were discovered in Lishui sag, East China Sea Basin. Geochemical proxies indicate that CO2 in these gas reservoirs is dominated by mantle-source volcano degassing. Although three volcanic events occurred in the formation of these reservoirs, the CO2 charging period is not clear. In this study, pure CO2, CO2-rich and H2O-rich fluid inclusions were found in healed micro-fractures and overgrowth zones of quartz in the reservoir of Lishui sag. The composition and density of two H2O-rich inclusions, two CO2-rich inclusions and twenty-three pure CO2 inclusions were determined by Raman quantitative analysis. The burial depth of CO2 fluid inclusion assemblages (FIA) can be determined by the trapping pressure constrained from Raman quantitative analysis and thermodynamic models. The combination of C-He isotope analyses, PT entrapment conditions of CO2-bearing inclusions, feature of fault activity and simulation of burial history permits to assign periods of CO2 fluid activity. The low-density (∼0.15 g cm−3) CO2 gas inclusions recorded an early-period of CO2 activity eruption to be related with the Late-Paleocene volcanic event. The mid-density (∼0.45 g cm−3) and high-density (∼0.75 g cm−3) CO2 inclusions were assigned to the Miocene volcanic event and to a later CO2 charging event during Pliocene.

Permanent CO2 Trapping through Localized and Chemical Gradient-Driven Basalt Carbonation.

Recent laboratory and field studies have demonstrated that basalt formations may present one of the most secure repositories for anthropogenic CO2 emissions through carbon mineralization. In this work, a series of high-temperature, high-pressure core flooding experiments was conducted to investigate how transport limitations, reservoir temperature, and brine chemistry impact carbonation reactions following injection of CO2-rich aqueous fluids into fractured basalts. At 100 °C and 6.3 mM [NaHCO3], representative of typical reservoir conditions, carbonate precipitates were highly localized on reactive mineral grains contributing key divalent cations. Geochemical gradients promoted localized reaction fronts of secondary precipitates that were consistent with 2D reactive transport model predictions. Increasing [NaHCO3] to 640 mM dramatically enhanced carbonation in diffusion-limited zones, but an associated increase in clays filling advection-controlled flow paths could ultimately obstruct flow and limit sequestration capacity under such conditions. Carbonate and clay precipitation were further enhanced at 150 °C, reducing the pre-reaction fracture volume by 48% compared to 35% at 100 °C. Higher temperature also produced more carbonate-driven fracture bridging, which generally increased with diffusion distance into dead-end fractures. In combination, the results are consistent with field tests indicating that mineralization will predominate in buffered diffusion-limited zones adjacent to bulk flow paths and that alkaline reservoirs with strong geothermal gradients will enhance the extent of carbon trapping.

Geochemical features and genesis of continental cobalt-rich ferromanganese crusts

Mass cobalt-rich ferromanganese microcrusts and nodules similar in morphology and chemical composition to cobalt-rich ferromanganese deep-ocean crusts were found in Cenozoic volcanic rocks in southern Primorye. Research has shown that ore genesis of this type is genetically related to argillization and destruction of siliceous rocks by CO2-rich fluids, which is confirmed by experimental data on carbon erosion of iron-containing materials. Two types of this fluid ore genesis are recognized: (1) relatively high-temperature (vapor-condensate), related to late volcanic processes and fracture gas infiltration, and (2) low-temperature (vapor-liquid-condensate), controlled by degassing followed by carbon mobilization (gasification). Primarily colloidal ferromanganese segregations have high contents of Co, Ni, Pb, Cu, and Ce, typical of oceanic ore genesis. Regardless of the concentrations of these metals in the protoliths, their contents in microcrusts are similar (n-10n wt.%). This indicates the same ore genesis mechanism and similar sorption properties of the colloidal ferromanganese material formed. Barium- and cerium-rich ferromanganese microcrusts and nodules are abundant. Condensed drops of iron-containing platinum were found in apobasaltic nickel-rich ferromanganese segregations. There is a cerium paradox expressed as a minimum or a total lack of cerium among rare-earth phosphates associated with ferromanganese microcrusts. Fluid destruction and oxide metallization of ocean-floor basalts are assumed to be the main source of metals for oceanic ferromanganese crusts and nodules.

Pargasite in fluid inclusions of mantle xenoliths from northeast Australia (Mt. Quincan): evidence of interaction with asthenospheric fluid

Three spinel lherzolite xenoliths from Mt. Quincan (Queensland, northeastern Australia) were studied with special attention to their enclosed fluid inclusions. The xenoliths are deformed, have porphyroclastic textures and overall show very similar petrographic features. The only significant difference is manifested in the abundance of fluid inclusions in the samples, mostly in orthopyroxene porphyroclasts. Xenolith JMTQ11 is fluid inclusion-free, whereas xenolith JMTQ20 shows a high abundance of fluid inclusions (fluid inclusion-rich). Xenolith JMTQ45 represents a transitional state between the previous two, as it contains only a small amount of fluid inclusions (fluid inclusion-bearing). Previous studies revealed that these xenoliths and the entrapped fluid inclusions represent a former addition of a MORB-type fluid to the pre-existing lithosphere, resulting from asthenosphere upwelling. There is a progressive enrichment in LREE, Nb, Sr and Ti from the fluid inclusion-free xenolith through the fluid inclusion-bearing one to the fluid inclusion-rich lherzolite. This suggests an increase in the extent of the interaction between the fluid-rich melt and the lherzolite wallrock. In addition, the same interaction is considered to be responsible for the formation of pargasitic amphibole as well.The presence of fluid inclusions indicates fluid migration at mantle depth, and their association with exsolution lamellae in orthopyroxene suggests fluid entrapment following the continental rifting (thermal relaxation) during cooling. A series of analyses, including microthermometry coupled with Raman spectroscopy, FTIR hyperspectral imaging, and Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) was carried out on the fluid inclusions. Based on the results, the entrapped high-density fluid is composed of 75–89 mol% CO2, 9–18 mol% H2O, 0.1–1.7 mol% N2 and ≤0.5 mol% H2S with dissolved trace elements (melt component).Our findings suggest that the metasomatic fluid phase could have been either a fluid/fluid-rich silicate melt released from the deeper asthenosphere, or a coexisting incipient fluid-rich silicate melt. Further cooling, possibly due to thermal relaxation and the upward migration of the fluid phase, caused the investigated lherzolites to reach pargasite stability conditions. We conclude that pargasite, even if only present in very limited modal proportions, can be a common phase at spinel lherzolite stability in the lithospheric upper mantle in continental rift – back-arc settings. Studies of fluid inclusions indicate that significant CO2 release from the asthenosphere in a continental rifting environment is resulting from asthenosphere upwelling and its addition to the lithospheric mantle together with fluid-rich melt – lherzolite interaction that leaves a CO2-rich fluid behind.

Fluid inclusion geochemistry and magmatic oxygen fugacity of the Wenquan Triassic molybdenum deposit in the Western Qinling Orogen, China

The Wenquan Mo deposit in Gansu Province, China, is one of the large porphyry Mo deposits in the Western Qinling Orogen that associated with the Triassic continental collision between the North China Block and South China Block. In this study, detailed fluid inclusions (FIs) analyses were performed on early stage quartz–biotite–K-feldspar veins, middle stage quartz–polymetallic sulfide veins and late stage sulfide–calcite veins. Four compositional types of FIs are observed, including pure CO2 (PC-type), CO2-H2O (C-type), H2O-NaCl (W-type), and daughter mineral-bearing polyphase (S-type, with no halite daughter mineral). The early stage quartz contains all the four types, which are mainly homogenized between 285 °C and 495 °C, with salinities ranging from 4.2 to 17.6 wt% NaCl eqv. However, in the middle stage, PC-type FIs are not observed, and S-, W- and C-types FIs yield homogeneous temperatures (Th) of 200–397 °C and salinities of 2.4–13.1 wt% NaCl eqv. Late stage quartz contains only W-type FIs with Th of 102–246 °C and salinities of 1.2–10.1 wt% NaCl eqv. Results indicate that the fluid-system evolved from an initial CO2-rich magmatic-hydrothermal fluid to a late CO2-poor meteoric fluid. In the early and middle stages, it is possible that heterogeneous FIs associations may have been trapped from a fluid immiscibility system. The dramatically decreasing percentage of PC- and C-types FIs in the ore stage indicates that CO2 escaped, which is possibly the result of earlier fluid immiscibility and phase separation. In addition, there are double peaks of Th in this stage that are obviously lower than in the early stage, showing that meteoric water flowed into and mixed with magmatic fluids, thereby cooling the hydrothermal system. These results suggest that metal precipitation resulted from fluid immiscibility, CO2 escape, and inflow of meteoric water occurred in the middle stage. The initial fluids of the Wenquan deposit are characterized by relatively high temperature, high salinity, high fO2, and are CO2-rich and NaCl-poor, indicating that the Wenquan deposit is a collisional-type porphyry Mo deposit.The oxygen fugacity (fO2) constrained by trace element compositions of zircons from the Wenquan Mo-bearing porphyry is uneven with ΔFMQ −10.3–+4.68 (average of −4.32), which could result from the input of mafic magma and its inhomogeneous mixing with felsic magma. Such values are still much higher than those of the adjacent contemporaneous Mo-barren Lujing, Baijiazhuang, Jiaochangba, Luchuba, and Zhongchuan granitic plutons, which confirms the affinity between Mo mineralization and the oxidized melt system and implies that the Wenquan porphyry has relatively high fO2 characteristics for its tectonic location and epoch. Fluid inclusion, initial mineralization depth (∼8.6 km), magma fO2, and the zircon high U/Yb ratio of the Wenquan Mo deposit strongly suggest that Mo mineralization was related to continental collision, rather than oceanic crust subduction. This study therefore proposes that the Wenquan Mo deposit was formed in a rapid crust uplifting setting caused by syn-orogenic crustal thickening followed by post-orogenic delamination of the lithospheric root during the Late Triassic.

Carbonation of mantle peridotite by CO2-rich fluids: the formation of listvenites in the Advocate ophiolite complex (Newfoundland, Canada)

The mantle section of the Advocate ophiolite (Newfoundland, Canada) contains unique outcrops of listvenite (magnesite-quartz), antigorite- and quartz-bearing talc-magnesite rock, and carbonated antigorite-serpentinite. This lithological sequence records the sequential carbonation of serpentinite by CO2-rich hydrothermal fluids. High Cr and Ni contents and preservation of Cr-spinel with a composition similar to that of Atg-serpentinite (molar Mg/Mg + Fe = 0.50–0.65; Cr/Cr + Al = 0.50–0.70), show that the Advocate listvenite and talc-magnesite rocks formed by carbonation of variably serpentinized mantle harzburgite. Replacement of lizardite by magnesite coeval with the breakdown of lizardite to antigorite + brucite and the lack of prograde olivine and magnetite in antigorite serpentinite and talc-magnesite rocks constrain the temperature of carbonation between c. 280 °C and 420 °C. Thermodynamic modelling of carbonation of serpentinite at 300 °C and 0.2–0.5 GPa accounts for the sequence of carbonated rocks in the Advocate complex. Phase relations and petrological observations indicate that the aqueous aSiO2 and aCO2 of the infiltrating CO2-rich fluid were buffered at the Atg-Tlc-Mgs and Qtz-Tlc-Mgs pseudo-invariant points, forming dominantly three-phase rocks by variable extents of carbonation at these pseudo-invariant points. Listvenites formed at large fluid-rock ratio when quartz became saturated in the fluid and precipitated along magnesite grain boundaries and in variably sized tensional veins.The whole rock Fe3+/Fetotal ratio of the Advocate carbonate-bearing sequence decreases with increasing whole rock carbon content, from 0.65–0.80 in brucite-bearing antigorite serpentinite to 0.10–0.30 in talc-magnesite rocks and listvenite. The whole rock iron reduction is associated with an increase in the ferrous iron content of magnesite and the formation of hematite and goethite, indicating a concomitant increase of the fluid oxygen fugacity. The sequence of carbonation reactions is uniquely preserved in three main growth zones characteristic of listvenite magnesite: (i) an inner zone of magnetite-bearing, Fe-poor, Mn-bearing magnesite formed by carbonation of lizardite, brucite and olivine from Atg-serpentinite; (ii) an outer zone of Fe-rich magnesite formed by carbonation of antigorite and in equilibrium with Fe-poor talc; and (iii) an outermost rim of Fe-poor magnesite formed by carbonation of talc.We propose that carbonation of the Advocate serpentinized mantle harzburgite occurred in a supra-subduction upper plate ophiolite by fluxing of slab-derived, CO2-rich fluids channelled along deep faults at the onset of accretion of the forearc basin (c. 300 °C, <0.5 GPa). The rather constant δ18O (11.0–14.4‰ V-SMOW) and relatively low δ13C (−8.9 to −5.0‰ V-PDB) of magnesite throughout the sequence of carbonated rocks in the Advocate complex is consistent with CO2-rich fluids derived from decarbonation or dissolution of organic carbon- and carbonate-bearing meta-sediments, such as those occurring in the underlying Birchy complex — the partially subducted continental margin of Laurentia. Carbonation of serpentinized oceanic or continental mantle lithosphere by reactive percolation of CO2-rich fluids derived from the slab in forearc settings may represent a significant carbon reservoir for the deep carbon cycle.

Timing of Secondary Hydrothermal Alteration of the Luobusa Chromitites Constrained by Ar/Ar Dating of Chrome Chlorites

Chrome chlorites are usually found as secondary phases formed by hydrothermal alteration of chromite deposits and associated mafic/ultramafic rocks. Here, we report the 40Ar/39Ar age of chrome chlorites separated from the Luobusa massive chromitites which have undergone secondary alteration by CO2-rich hydrothermal fluids. The dating results reveal that the intermediate heating steps (from 4 to 10) of sample L7 generate an age plateau of 29.88 ± 0.42 Ma (MSWD = 0.12, plateau 39Ar = 74.6%), and the plateau data points define a concordant inverse isochron age of 30.15 ± 1.05 Ma (MSWD = 0.08, initial 40Ar/36Ar = 295.8 ± 9.7). The Ar release pattern shows no evidence of later degassing or inherited radiogenic component indicated by an atmospheric intercept, thus representing the age of the hydrothermal activity. Based on the agreement of this hydrothermal age with the ~30 Ma adakitic plutons exposed in nearby regions (the Zedong area, tens of kilometers west Luobusa) and the extensive late Oligocene plutonism distributed along the southeastern Gangdese magmatic belt, it is suggested that the hydrothermal fluids are likely related to the ~30 Ma magmatism. The hydrothermal fluid circulation could be launched either by remote plutons (such as the Sangri granodiorite, the nearest ~30 Ma pluton west Luobusa) or by a similar coeval pluton in the local Luobusa area (inferred, not found or reported so far). Our results provide important clues for when the listwanites in Luobusa were formed.

Composition and source of fluids in high-temperature graphite-bearing granitoids associated with granulites: Examples from the Southern Marginal Zone, Limpopo Complex, South Africa

P-T conditions, fluid regime and carbon isotope composition of graphite and fluid inclusions from garnet-sillimanite-bearing leucocratic tonalites, trondhjemites and granites associated with orthopyroxene-bearing granulite metapelites in the Southern Marginal Zone (SMZ) of the Limpopo Complex, South Africa, are presented in the paper. Re-integrated compositions of perthitic alkali feldspars and antiperthitic plagioclase, as well as P-T and T-XCO2 phase equilibria modeling using PERPLE_X software indicate that the granitoids began to crystallize at temperatures of 900–940 °C and pressures of 7–9 kbar, and were equilibrated with a fluid phase with XCO2 > 0.5–0.6 as is recorded in dense fluid inclusions in quartz. A small fraction of a saline fluid accumulated during cooling only. Average δ13CPDB values for graphite (−6.52 to −8.65‰) and fluid inclusions (−2.50 to −5.58‰) from the granitoids differ substantially from the values, which have been obtained in previous studies for graphite from the surrounding SMZ metapelites. Isotope data thus indicate that fluids associated with the granitoid magmas of the SMZ originated from a source unrelated to the host metapelites. In terms of the “heavy” isotope signatures of carbon, the granitoids might carry fluids, which have been produced during devolatilization of mafic rocks (amphibolites) interlayered with hydrothermal carbonate veins in the adjacent granite-greenstone successions of the Kaapvaal craton that have been buried underneath the SMZ granulites during their exhumation. The cratonic rock could serve as a source for the trondhjemite magmas intruding the SMZ granulites. In this scenario, the studied granitoids crystallized from these magmas variously contaminated by metapelitic material. Graphite precipitated via reduction of CO2-rich fluid as the result of dissolution of pyrrhotite-rich metapelites in the magmas.

A magnetotelluric study from over Dharwar cratonic nucleus into Billigiri Rangan charnockitic massif, India

The electrical resistivity structure of the crust beneath the Dharwar craton in southern India was investigated by magnetotelluric method. In the present study, a northwest-southeast oriented 220 km long profile of 18 stations with a station spacing of ∼10–15 km is used. The profile extends from Dharwar cratonic nucleus in the north to Billigiri Rangan charnockitic massifs in the south. Time series data are processed to get the apparent resistivity and phase. The dominant geoelectric strike direction (75°) was calculated in a period range of 0.01–1000 s. The data are rotated to 75° strike direction. Two-dimensional inversion is carried out by using the non-linear conjugate gradient scheme for both apparent resistivity and phase. Our inversion results show a conductor in the northern side of the profile and two distinct prominent conductors in southern part of the profile. The mid-lower crust in southern part of the profile shows less resistive (<300 ohm-m) nature in the depth range of 20–50 km and is related with the Chitradurga shear zone and Billigiri Rangan charnockite massif. These zones were interpreted as CO2 flushed terranes. Regional-scale carbonation occurred in Late Archaean is associated with Chitradurga shear zone and in Late Proterozoic is associated with Salem-Attur shear zone. The CO2 rich fluids derived during that time might have exhausted in dehydration reactions. Later events such as the Indian plate passing over several hotspots and the metasomatized fluids associated with the Cretaceous-Tertiary magmatism in the region is the reason for observed low resistivity near Billigiri Rangan massif and surrounding regions in the south.

Seismic evidence for depth-dependent metasomatism in cratons

The long-term stability of cratons has been attributed to low temperatures and depletion in iron and water, which decrease density and increase viscosity. However, steady-state thermal models based on heat flow and xenolith constraints systematically overpredict the seismic velocity-depth gradients in cratonic lithospheric mantle. Here we invert for the 1-D thermal structure and a depth distribution of metasomatic minerals that fit average Rayleigh-wave dispersion curves for the Archean Kaapvaal, Yilgarn and Slave cratons and the Proterozoic Baltic Shield below Finland. To match the seismic profiles, we need a significant amount of hydrous and/or carbonate minerals in the shallow lithospheric mantle, starting between the Moho and 70 km depth and extending down to at least 100–150 km. The metasomatic component can consist of 0.5–1 wt% water bound in amphibole, antigorite and chlorite, ∼0.2 wt% water plus potassium to form phlogopite, or ∼5 wt% CO2 plus Ca for carbonate, or a combination of these. Lithospheric temperatures that fit the seismic data are consistent with heat flow constraints, but most are lower than those inferred from xenolith geothermobarometry. The dispersion data require differences in Moho heat flux between individual cratons, and sublithospheric mantle temperatures that are 100–200 °C less beneath Yilgarn, Slave and Finland than beneath Kaapvaal. Significant upward-increasing metasomatism by water and CO2-rich fluids is not only a plausible mechanism to explain the average seismic structure of cratonic lithosphere but such metasomatism may also lead to the formation of mid-lithospheric discontinuities and would contribute to the positive chemical buoyancy of cratonic roots.

Modulation of magmatic processes by CO2 flushing

Magmatic systems are the engines driving volcanic eruptions and the source of fluids responsible for the formation of porphyry-type ore deposits. Sudden variations of pressure, temperature and volume in magmatic systems can produce unrest, which may culminate in a volcanic eruption and/or the abrupt release of ore-forming fluids. Such variations of the conditions within magmatic systems are commonly ascribed to the injection of new magma from depth. However, as magmas fractionating at depth or rising to the upper crust release CO2-rich fluids, the interaction between carbonic fluids and H2O-rich magmas stored in the upper crust (CO2 flushing), must also be a common process affecting the evolution of subvolcanic magma reservoirs. Here, we investigate the effect of gas injection on the stability and chemical evolution of magmatic systems.We calculate the chemical and physical evolution of magmas subjected to CO2-flushing using rhyolite-MELTS. We compare the calculations with a set of melt inclusion data for Mt. St. Helens, Merapi, Etna, and Stromboli volcanoes. We provide an approach that can be used to distinguish between melt inclusions trapped during CO2 flushing, magma ascent and decompression, or those affected by post-entrapment H2O-loss. Our results show that CO2 flushing is a widespread process in both felsic and mafic magmatic systems. Depending upon initial magma crystallinity and duration of CO2 input, flushing can either lead to volcanic eruption or fluid release. We suggest that CO2 flushing is a fundamental process modulating the behaviour and chemical evolution of crustal magmatic systems.